1. Field of the Invention
The present invention relates to an extractant and an extraction process. More particularly, the invention relates to an extractant useful for extracting metal ions from an aqueous solution.
2. Description of the Related Art
The platinum group metals (PGMs) are exceedingly rare in nature with average crustal quantities of only a few fractions of a part-per-million (ppm). Only a few large deposits of platinum group metals are known to exist and these deposits are located in South Africa and Russia. Smaller quantities of PGMs are obtained from the anode slimes produced during the electrorefining of nickel and from a few small mineral deposits variously located around the world. The growing demand for platinum group metals and other precious metals in high-technology applications has generated a demand for methods and processes for recycling these materials, especially from spent catalysts and from electronic scrap. It is important that PGMs be recycled because of the limited natural supply of these elements.
PGMs are used as catalysts in the chemical and automotive industry and as oxidation resistant coatings in the electronics sector. Many of these uses, especially the catalytic uses, are ideal for recovering the PGMs because the spent catalyst may be easily processed for metal recovery as part of the disposal process for the spent catalysts. Therefore, there is an interest in improving the methods and processes used for recovering PGMs by making these methods and processes faster and more efficient.
Using traditional smelting techniques to treat materials such as spent catalysts or electronic scrap is not always effective due to the refractory nature and low precious metal content of the spent catalysts and electronic scrap. Leaching spent catalysts and other scrap materials with acidic chloride solutions containing an oxidizing agent is effective in removing the precious metals and has the added advantage of preserving valuable substrates for recycling. However, the recovery and subsequent separation of precious metals, including PGMs, from the chloride feed liquors constitutes a difficult problem because these liquors often contain only low levels of precious metals (ppm levels) but high levels (on the order of grams per liter) of base metals such as iron, copper, zinc, tin, and nickel. Moreover, the volumes of solutions generated from the acid recovery of precious metals from spent catalysts and the like are large compared to the volume of highly concentrated solutions generated from typical precious metal refining. Classical precipitation techniques are inefficient when applied to such solutions and these classical techniques are being replaced by modern separation methods such as solvent extraction processes.
Solvent extraction, sometimes referred to as liquid ion exchange extraction, takes place in two steps. In the first step, the extraction step, a dilute aqueous feed solution containing the metal ion to be recovered is mixed with an immiscible hydrocarbon carrier containing an extractant dissolved therein. When the metal ion contacts the extractant, a metal complex is formed that migrates to the organic phase. In the second step, the stripping step, the “loaded” organic phase, which has been separated from the aqueous feed solution, is mixed with another aqueous solution of a stripping agent (e.g., sulfuric acid) wherein the metal ion passes to the aqueous stripping phase. Therefore, the extraction process converts a dilute feed solution of metal ions into a highly concentrated solution of the precious metal ions from which the metals may be more readily recovered, e.g., by electrolysis. The barren organic phase may then be recycled through the system if desired.
Monoquaternary amines have been somewhat effective in recovering platinum group metals from acidic process streams containing base metals such as nickel and cobalt. Monoquaternary amines contain a positively charged nitrogen atom having four groups bonded to the nitrogen atom, and another atom or substituent that neutralizes the positive charge, typically a hydroxide or a chloride. However, the monoquaternary amines currently used in separation processes are not highly selective, although some selectivity has been achieved by modifying the substituent groups on the quaternized nitrogen atom or by carefully selecting the organic solvent used as a diluent. A monoquaternary amine currently being used in extracting PGMs is N-methyl-N,N-dioctyl-1-octanaminium chloride. (Available as ALIQUAT 336, a registered trademark of the Henkel Corporation of Germany).
In extracting PGMs from an aqueous acidic solution using a monoquaternary amine, the monoquaternary amine is first dissolved in a predominately water-immiscible or organic phase, such as 1-octanol. The aqueous and the organic solutions are then intimately mixed to allow the dissolved quaternary amine salt to form an ion pair with the PGM and transfer the desired PGM species from the aqueous phase into the organic phase. The two phases can then be separated and the extracted anion recovered from the organic phase.
A monoquaternary amine has also been adsorbed onto an inert polymeric support, while still retaining its desirable anion exchange properties, by making a slurry of the quaternary amine in methanol with resin beads and slowly removing the methanol using a rotary evaporator, leaving the monoquaternary amine adsorbed onto the surface of the resin beads. Since the monoquaternary amine is not covalently attached, but only held by weak Van der Waals attractions, the selectivity in extracting PGMs is similar to that of the free monoquaternary amine. In extraction systems, it is preferable for the extractant to be immobilized on a solid support because it eliminates the organic solvent, thereby making processing simpler with fewer environmental concerns.
What is needed is a more selective extractant compound for extracting precious metals, such as PGMs, from acidic solutions. It would be an advantage to provide a method for designing an efficient extraction compound having a high selectivity for a particular precious metal.